Section 7: Bonded and Unbonded Concrete Overlays

7.1 Introduction

Bonded concrete overlay (BCO) consists of a 2-in. to 8-in.
thick concrete layer placed on top of the existing concrete pavement
with operations conducted to ensure full bond between new and old concrete
layers. A BCO with a minimum thickness of 4 in. is one of the most
cost-effective ways of enhancing structural capacity of under-designed
pavements by reducing deflections and extending service life. A
BCO with a thickness of less than 4 in. is typically used to restore
pavement surface characteristics, such as ride and friction.

The department maintains many miles of thin PCC pavement that
have exceeded their design traffic projection and are still in reasonably
good condition. The use of BCO is based on the fundamental design
assumption that the old and new concrete layers behave as a monolithic
layer. Providing full bond is of the utmost importance. During construction,
specific steps are taken to enhance and ensure the full bond between
old and new concrete as discussed in Chapter 10.

Bonded concrete overlays over jointed concrete pavements are
difficult to construct because all joints must be matched. CRCP-bonded
concrete overlays have been constructed and have performed successfully
in several districts but have not been used widely throughout the
state. Districts considering a bonded concrete overlay can contact MNT
– Pavement Asset Management, Pavement Analysis & Design Branch,
for assistance.

Unbonded concrete overlay consists of a concrete layer (5
in. or greater) on top of an existing concrete with a HMA interlayer
to separate new overlay and existing concrete. An unbonded overlay
is a feasible rehabilitation alternative for PCC pavement for practically
all conditions. These types of rehabilitation methods are most cost-effective
when the existing pavement is badly deteriorated because a reduced
amount of repairs were made to the existing pavement prior to constructing
the unbonded concrete overlay.

Unbonded CRCP concrete overlays may be used over CRCP, jointed
concrete pavement (CPCD), or jointed reinforced concrete pavement
(JRCP). Unbonded CRCP overlay uses the same design procedure as
new CRCP pavements. This use of unbonded CRCP overlay can be credited
for contributing to the structural capacity of the existing concrete
pavement and results in a thinner concrete pavement design than
required for CRCP constructed on a new location.

7.2 AASHTO Overlay Thickness Design for Bonded Overlays

The equation below is used to calculate the overlay thickness
to increase structural capacity to carry future traffic. The designer
can also use the DARWin® 3.1 program and select the “Overlay Design”
Module and “Bonded PCC Overlay of PCC Pavement.”

Dol = Df - Deff

Where:

Dol = required slab thickness of overlay,
in.

Df = slab thickness to carry future
traffic, in.

Deff = thickness of existing slab,
in.

The slab thickness, Df, is determined
for the design traffic as if it were built as a new pavement on the
prepared base. Use the design procedure in Sections 3 or 4 for the
pertinent pavement type. Some existing pavements might not have
stabilized bases, and the k-value should be evaluated using FWD
or DCP.

For BCO design, the condition of the existing pavement is
one of the most important factors. If the pavement condition is
deteriorated in the form of punchouts and deteriorated cracks, BCO
may not be a good alternative.

Punchouts are the only structural distresses in CRCP; the
number of punchouts per mile is a good indication of the structural
condition of the existing pavement. If there are more than 12 punchouts per
mile, then the pavement is in poor structural condition and may
not be a good candidate for BCO.

The number of deteriorated transverse cracks per mile is the
next item to be surveyed. Even though some transverse cracks may
appear to be deteriorated, quite often they are structurally in
good condition. In Texas, it is rare to observe deteriorated transverse
cracks, except for spalled cracks. Most spalled cracks are not necessarily
structurally deficient. Based on the research findings, it is recommended
that only transverse cracks much wider than normal, along the entire
length of the transverse crack and across the entire lane width,
should be counted as “deteriorated transverse cracks.”

The number of patches per mile and evidence of pumping should
be recorded. In Texas’ old concrete pavements, typically, the base
was not stabilized and pumping resulted. Durability related problems,
such as D-cracking and ASR cracking, should be noted and their severity
recorded. Overall, it has been quite rare to observe durability-related
problems in concrete pavement in Texas.

7.2.1 Joints and Cracks Adjustment Factor, Fjc

The Joints and Cracks Adjustment Factor, Fjc,
accounts for the extra loss in the present serviceability index
(PSI) caused by deteriorated reflection cracks in the overlay. Deteriorated
reflection cracks develop due to unrepaired deteriorated joints,
cracks, and other discontinuities in the existing slab prior to
the overlay.

A deteriorated joint or slab will rapidly reflect through
an overlay and contribute to loss of serviceability. Therefore,
full-depth repair is recommended on all deteriorated cracks and
any other major discontinuities in the existing pavement prior to
overlay. The target Fjc is 1.00.

If it is not possible to repair all the deteriorated areas,
use the total number of unrepaired deteriorated joints, cracks,
punchouts, and other discontinuities per mile in the design lane
to determine the Fjc from Figure 8-5.

7.2.2 Durability Adjustment Factor, Fdur

The Durability Adjustment Factor, Fdur,
adjusts for an extra loss in PSI of the overlay when the existing
slab has durability problems, such as D-cracking or reactive aggregate
distress. Fdur is determined using historical
records and condition survey data. Table 8-5 shows the durability adjustment
factor.

7.2.3 Fatigue Damage Adjustment Factor, Ffat

The Fatigue Damage Adjustment Factor, Ffat,
adjusts for past fatigue damage in the slab. It is determined by
observing the extent of punchouts (CRCP) that may be caused primarily
by repeated loading. Table 8-6 shows the fatigue damage adjustment
factor.

7.2.4 Reinforcement Design

Reinforcement should be placed at a depth that provides a
minimum concrete cover of 3 in. When BCO thickness is 3 in. or less,
reinforcement in the form of longitudinal steel is not recommended. For
thinner overlays, fibers have been successfully used.

The design engineer will determine the reinforcement bar size
and number of longitudinal steel, tie bars, and transverse steel.
The performance of CRCP depends on the percentage of longitudinal steel.
Failing to place longitudinal steel in a BCO 4 in. or thicker will
effectively reduce the percentage of longitudinal steel in the combined
slab, which could increase steel stresses and make transverse crack
widths larger. See the recommended reinforcing steel percentage
and vertical location in Table 8-7.

7.3 AASHTO Overlay Thickness Design for Unbonded Concrete Overlays

The 1993 AASHTO Guide for Design of Pavement Structures is
recommended for unbonded concrete overlay thickness design. The
designer can use the AASHTO DARWin® 3.1 program and select the “Overlay
Design” Module and “Unbonded PCC Overlay of PCC Pavement.”

The reinforcing steel placements are the same as new CRCP
when unbonded overlay is 7 in. or thicker. When unbonded overlay
is less than 7 in., use longitudinal reinforcement at about 0.6%
of concrete cross-sectional area. The design engineer will determine
the steel bar size and quantities for longitudinal bars, tie bars,
and transverse bars, and consult with the Pavement Analysis
& Design Branch of MNT – Pavement Asset Management.